As a polarizing plate for use in liquid crystal display devices, a polarizing plate manufactured by forming a triacetyl cellulose (to be abbreviated as TAC hereinafter) film as a protective film on one side or both sides of a monoaxially oriented polyvinyl alcohol (to be abbreviated as PVA hereinafter) film colored with iodine as a polarizer, a polarizing plate manufactured by forming an acrylic resin coating layer on one side of a polarizer, or a polarizing plate manufactured by forming a norbornene or polycarbonate retardation film on one side of a polarizer in place of TAC is used. However, as the above polarizing plates are classified into a group of absorption type polarizing plates which transmit only light in the transmission-axis direction of the polarizing plate and absorb light of other components, a liquid crystal display device having a light transmittance of 50% (maximum light transmittance of 46% because the internal surface reflectance is 4%) under ideal conditions and increased brightness and making efficient use of a backlight is targeted.
Liquid crystal display devices are used in a wide variety of products from small-sized products such as calculators, wrist watches, mobile phones and digital cameras to middle-sized products such as PDA, car navigation systems and POS systems and large-sized products such as liquid crystal monitors for personal computers and liquid crystal TVs. Demand for members having higher quality and more functions for use in the liquid crystal display devices is growing.
A reflection type polarizing plate making use of optical interference is available as a polarizing plate having a brightness increasing function. U.S. Pat. Nos. 3,610,729 and 5,486,949 disclose a polarizer making use of the interference of a multi-layer film having birefringence and a method of separating polarization with an oriented multi-layer film consisting of two different polymer films which differ in refractive index. Journal of Applied Physics, vol. 37, pp. 4389, 1998 proposes a method of separating polarization by making use of a simple polymer blend though its principle is the same as above. Recently, WO 2005/8302 has reported a method making use of a fiber in place of the polymer blend. A reflection type polarizing plate in the polarization separation system reflects a non-transmitting polarization component. The reflected light is multiply reflected by a diffusion reflective film installed on the back light side of a liquid crystal display device repeatedly, thereby making it possible to take out light corresponding to light in the transmission-axis direction of the polarizing plate again and to realize a transmittance of 60% or more. However, when the reflection type polarizing plates are installed before and after the liquid crystal cell, color cannot be displayed accurately due to the multiple reflection of light reflected in the liquid crystal cell. Therefore, it is used only on the backlight side. As described in the above WO2005/8302 and at page 13 of the monthly magazine “Display” issued in April 2005, to realize a reflection type polarizing plate, the refractive index of a polymer to be blended and the refractive index of a substrate as a bulk must be made exactly the same, the shapes and arrangements of the blend polymer and the fiber must be controlled precisely, it is extremely difficult to achieve the same degree of polarization (99.9% or more) as that of the current polarizing plate, and the contrast of a liquid crystal display device cannot be maintained at a satisfactory level. Therefore, the reflection type polarizing plate is used in combination with an absorption type polarizing plate to attain only the function of increasing brightness in the current situation. In this case, the reflection type polarizing plate is not called “polarizing plate” but treated as a brightness increasing film and actually marketed under the trade name of D-BEF from 3M Co., Ltd. However, as it is necessary to secure polarization characteristics over a wide visible range even in D-BEF, 400 to 800 layers in total are assembled together. It is technically difficult to carry out the control of the thickness of each layer and the assembly of several hundreds of polymer films and further to control the refractive index of each layer.
Since durability required for the polarizing plate is extremely high, an absorption type polarizing plate using iodine is unsatisfactory in terms of durability, especially moist heat resistance, and the service life of a liquid crystal display device comprising this polarizing plate is limited by the characteristic properties of the polarizing plate.
To improve the durability, the monthly magazine “Display” issued in April, 2005 reports at page 39 a dye-based polarizing plate comprising a dichroic dye as a polarizer in place of iodine. Although the durability of the polarizer itself is improved as compared with that of a polarizing plate comprising iodine adsorbed thereto and aligned, a TAC film which is the protective film of the polarizing plate shrinks and applies shrinkage stress to a liquid crystal panel with the result of the deterioration of image quality or the corrosion of the TAC film itself.
As described above, to improve the transmittance of the polarizing plate, a reflection type polarizing plate is used, or an absorption type polarizing plate and a reflection type polarizing plate, that is, two brightness increasing films are used so as to increase the brightness of a liquid crystal display device. When a reflection type polarizing plate is used alone, the contract of the liquid crystal display device may not be maintained at a satisfactory level due to a low degree of polarization, and when a brightness increasing film is used alone, it cannot have a polarizing function and has a large number of technically difficult points, thereby boosting its costs. Therefore, a polarizing plate alone having a high degree of polarization and a high transmittance (having a brightness increasing function) is not obtained yet.
When a TAC film is used as a polarizing plate protective film, the improvement of the durability of the polarizing plate has not been observed until now.
In a reflection type or semi-transmission reflection type small- to medium-sized liquid crystal display device, a λ/4 retardation film is joined to a polarizing film at about 45°. This is essential in order to obtain display by reflection. The polarizing film and the retardation film are each cut into a chip having a specific angle and these chips are joined together. As for the oblique stretching of the polarizing film, JP-A 2003-207622 and JP-A 2004-144838 disclose a technology for a polarizing film having a transmission axis in an oblique direction by using a polarizer composed of a PVA film which is obliquely stretched. However, to stretch the polarizer composed of a PVA film, the draw ratio must be 4 to 8 times and a bulky apparatus is required. In the oblique stretching of the film, as the stretched film is biaxially oriented, the degree of orientation is low, thereby making it impossible to obtain the same polarization performance as that of a polarizing film composed of a polyvinyl alcohol film which is monoaxially stretched in a longitudinal direction. Since a TAC film is used as the protective film of the polarizer of a polarizing film, problems such as the deterioration of image quality by shrinkage which depends on the TAC film and the corrosion of the TAC film itself still remain.
In the reflection type or half-transmission reflection type liquid crystal display device, a liquid crystal display device is constructed by cutting a polarizing film into a chip having a specific angle and joining it. Therefore, a roll of the polarizing film cannot be joined to a roll of a retardation film, and the process becomes very complicated. Although a technology for stretching a polarizer composed of a PVA film obliquely is now under study, a bulky apparatus is required and the same polarization performance as that of a polarizing film composed of a monoaxially oriented polyvinyl alcohol film cannot be obtained. Therefore, a liquid crystal display device comprising a PVA polarizer is not implemented yet. When a TAC film is used as the protective film of a polarizing film, the improvement of the durability of the polarizing film has not been observed until now.
As a latent problem in the liquid crystal display device, an image displayed by the device changes according to the view direction. One of the causes of this is a polarizing plate. That is, the polarizing plate is made of a monoaxially oriented film and this is caused by the change of the angle formed by the absorption axes of two polarizing plates arranged in a cross nicol state. As another cause, when the image is seen from above the polarizing plate, or when it is seen from a plane orthogonal to the polarizing plate and a plane including the transmission axis or absorption axis of the polarizing plate, the angle formed by the absorption axes of the two polarizing plates is fixed at 90°, and black can be formed by the polarizing plates. However, when seen from the plane orthogonal to the polarizing plate and a plane not including the transmission axis and the absorption axis of the polarizing plate (especially 45° direction), the angle formed by the absorption axes of the two polarizing plates is larger than 90° by changing the angle of observation, thereby causing light passage.
To compensate for a defect in view angle characteristics caused by the polarizing plate, a technology for inserting a biaxial retardation film is disclosed by JP-A 10-142423 and Japanese Patent No. 3526830. With this technology, a wide view angle polarizing plate which has the function of making the polarization state of light passing through a polarizer when light is input obliquely equal to the polarization state when light is input vertical to the retardation film, compensates for a change in the optical properties of the polarizer and can provide a liquid crystal display device having a wide view angle can be obtained. However, the biaxial retardation film for realizing these characteristic properties must control Nz which is birefringence and therefore, a special and difficult processing technology for stretching a monoaxially oriented film in the thickness direction of the film by shrinking a thermally shrinkage film which is joined to both sides of the monoaxially oriented retardation film is required. Therefore, the retardation film has problems such as low productivity due to low yield, a very expensive material, and increases in the number of the materials which must be used to join it to a polarizing plate and the number of processes.
To improve the view angle characteristics of the polarizing plate, the technology for inserting a biaxial retardation film is used. However, the difficulty of the process, and increases in the number of materials and the number of processes boost the costs of members, and a method for improving the view angle characteristics with the polarizing plate alone is not realized yet.